1. Field of the Invention
The present invention relates to fire-retardant sheet material and especially to such sheet material obtained from a polyethylene composition and intended for use in the construction and building industries.
2. Description of the Prior Art
Fire-retardant plastic compositions are known in the art. The polymer of such compositions is often polyvinyl chloride, or a related polymer, or the compositions contain halogenated compounds as flame-retardant agents. Although such compositions may have commercially-acceptable properties, there are concerns about flame-retardant compositions that contain halogenated matter, especially with respect to the toxicity and corrosivity of the decomposition products that may be formed when articles fabricated from such fire-retardant compositions are subjected to heat and/or fire.
Compositions, with fire-retardant properties, that do not contain halogenated materials are also known. Such compositions may be based on polyolefins, polyamides or the like and contain filler materials e.g. so-called alumina trihydate (Al(OH).sub.3), magnesium hydroxide, calcium carbonate, antimony trioxide, clay and the like. Such compositions are often used in the wire coating industry.
Sheet structures are used in the construction and building industries. For example, such structures may be used as ceiling tiles, wall panels, vertical blinds or the like. It is preferable and/or necessary that such sheet structures have flame-retardant properties. In particular, it is advantageous for such structures to be capable of meeting the requirements of a burn test conducted according to the procedures of ASTM E-84, especially the characteristics of Flame Spread Index (FSI) and Smoke Density (SD). Such characteristics may be specified in governmental regulations and/or be included in specifications of other organizations, e.g. the Underwriters Laboratories of Canada and the U.S.A., and be embodied in building codes or guidelines, or the like.
A compatibilized filled polyolefin composition comprising a hydroxyl-containing filler encapsulated in a polyolefin having a relatively high melt index and subsequently compounded with a polyolefin having a relatively low melt index is disclosed in Canadian Pat. No. 884 935 of N. G. Gaylord, issued Nov. 2, 1981. The encapsulation of the filler may be carried out in the presence of an ethylenically unsaturated carboxylic acid or anhydride under conditions that generate free radicals.
In Japanese patent application No. 55 112 248, published Aug. 29, 1980, H. Nakae et al disclosed compositions comprising 50-300 parts of particulate inorganic compound, per 100 parts of polyolefin, and 1-50 parts of maleic polymer per 100 parts of inorganic compound. Polyethylene having a density of 0.910-0.945 g/cm.sup.3 and a melt index of 0.01-2.0 dg/min is the preferred polyolefin. Maleic polymer was defined as polymer containing succinic acid groups, especially maleic polybutadiene and polypropylene graft modified with maleic anhydride. The maleic polymers were stated to be of relatively low molecular weight, preferably 1000-5000, and to melt and soften at less than 100.degree. C.